Various processes for the manufacture of microspheres of small diameter in the range 40-60 micrometers and with somewhat larger microspheres having a diameter in the range 0.25 to 5.0 millimeters have been described in the prior art, for example Japanese Published Patent Application Number 57-84731 and U.S. Pat. No. 4,746,468.
U.S. Pat. Nos. 3,491,492, 4,068,718, 4,427,068 and 4,713,203 all describe processes for manufacture from clay or bauxite material substantially spherical ceramic particles or pellets in the size range 0.25-5.0 millimeters, primarily for use as proppants. Apart from U.S. Pat. No. 4,713,203 which utilizes naturally occurring bauxite fines as feedstock the other prior art documents describe the use of relatively coarse clay or bauxite particles. U.S. Pat. No. 4,427,068 does imply however that expensive grinding of calcined clays or bauxite may be employed to produce particle sizes less than 15 micrometers.
Each of U.S. Pat. Nos. 3,491,492, 4,068,718, 4,427,068 and 4,713,203 is concerned with the manufacture of proppants for hydraulic fracturing of subterranean formations and each requires the physical formation of pellets by agglomeration in a rotary pelletizer or the like, with or without a binder. Subsequently calcining of the green pellets is usually carried out in a rotary calcining kiln.
Previous attempts to produce spherical proppants by spray drying have produced rounded non spherical particles characterized by a hollow recess similar in appearance to a mushroom cap. This shape has been ascribed to aerodynamic deformation of the slurry droplet in the hot gas stream before the particle dried.
Metal-matrix composites (MMCs) consist of a matrix of a metal or alloy into which one or more second phases have been incorporated with the air of reinforcing for improved properties. After many years of research, these materials are now becoming commercially available. The enhancement in properties over those of the matrix material which can be achieved include:
improvement in strength at ambient and high temperatures, PA0 improvement in stiffness, PA0 improvement in fatigue strength, PA0 improvement in wear resistance, PA0 reduction in coefficient of thermal expansion. PA0 matrix metal or alloy, PA0 content of the reinforcing phases, PA0 chemistry, geometry, distribution and orientation of the reinforcing phases, PA0 nature of the interface between the reinforcing phases and the matrix material, PA0 manufacturing method, PA0 thermo-mechanical history. PA0 preparing a dispersion of bauxite or bauxitic clay; PA0 classifying the dispersed bauxite particles to recover the ultrafine fraction; PA0 adding small quantities of water soluble salts, mineral compositions or organometallic complexes to control the microsphere surface chemistry so as to enhance the wetting and dispersion of the microsphere and improve its ability to bonding strongly with the matrix materials in use; PA0 spray drying the dispersion to produce green microspheres of a predetermined mean particle diameter, and PA0 subjecting said green microspheres to calcination and sintering to produce microspheres having a size within the range 0-100 micrometers, preferably from 1 to 50 micrometers and most preferably less than 30 micrometers, said microsphere being characterised by a substantially solid form having a pycnometric density substantially falling in the range 3.2 to 3.9 g/cm.sup.3, a BET surface area substantially falling in the range 0.05 to 0.5 m.sup.2 /g and a crystal grain size less than 4 micrometers. PA0 the need for expensive pregrinding is eliminated PA0 the need for expensive precalcination is eliminated PA0 high strength of the green microspheres is obtained without the addition of a binder PA0 the very high surface area of the feed material makes it highly reactive. This leads to a reduction in sintering time, hence in energy consumption PA0 an exceptionally high degree of uniformity in the composition of the microspheres PA0 1. The very fine particle size of the bauxite or bauxitic clay feedstock allows an exceptionally high degree of uniformity of blend in the manufacture of the green microsphere. PA0 2. The green microsphere comprising a multitude of ultrafine particles has numerous points of contact between the particles, and it is at these points that sintering will be initiated. PA0 3. It has been found that the extremely intimate dispersion of minerals and thus of constituent elements is conducive to a high degree of reactivity within each microsphere as it is heated. This leads to a correspondingly intimate dispersion of microcrystallites on sintering. The microspheres thus exhibit very high strength. It is well known in the art that ceramic products with a fine microcrystalline structure are extremely strong. PA0 4. The ultrafine particle size of the constituted minerals, combined with the small target size of the manufactured green microspheres, permits the use of the simple spray drying process to form particles of substantially spherical shape. Previous experience with the same material in spray drying to form larger particle diameters resulted in rounded but not spherical particles which were characterised by a shape of mushroom cap appearance. This shape has been ascribed to the aerodynamic deformation of the slurry droplet before it dried in the hot gas stream of the spray dryer. It has now been found that when the droplet size is restricted to a particle size less than about 100 micrometers, these deformities in shape substantially disappear. Scanning electron microscopy reveals that sphericity of particles under about 70 micrometers improves dramatically, with particles in the range from about 5 to 45 micrometers being almost perfectly spherical. It is believed that the higher rate of surface tension to mass in the fine droplets overcomes the tendency to form non-spherical particles. PA0 5. Since the only mechanism employed in the formation of the green microspheres from the ultrafine particles is the removal of hygroscopic moisture by drying, it will be apparent that the optimum benefit has been obtained from the extremely high interparticulate Van der Walls's forces. This leads to near maximum densification of the particles without the need for an expensive binder. PA0 6. The spray drying process allows relatively simple control over the particle size of the green microspheres. Choice of such appropriate spray drying parameters such as slurry solids concentration, slurry viscosity rate of introduction of inlet and outlet temperature and spray head type and configuration permit this control to be achieved. PA0 7. The choice of the calcination and sintering process is an important element in the large scale commercial production of ceramic microspheres from bauxite or bauxitic clay. Although the small scale production by firing in loosely packed beds is feasible, the process must be interrupted at a relatively high temperature to allow the bed to be stirred and to prevent the particles sintering together at the temperature chosen for optimum densification of the individual particles. This is clearly not feasible on a commercial scale. The small size of the green microspheres and their free-flowing nature precludes the economic and practical use of conventional rotary kilns as unacceptable losses of finer microspheres would occur. It was considered that the use of modern stationary calciners such as the so-called flash and gas suspension calciners and which are successfully used for the calcination of such materials as fine aluminum trihydroxide would not be suitable for the calcination and sintering of the ceramic microspheres. It was though that the very steep temperature gradient and very short residence times typical of such apparatus would lead to the shattering of the microspheres due to the inability of hygroscopic and chemically bound moisture to diffuse from the microspheres at sufficiently rapid rate. It was also thought that the turbulent nature of the hot gases in the calcination and sintering zone of the calciner that the microspheres would sinter together. Surprisingly, it was found that the amount of particle degradation in the product was low. It was also surprisingly found that few of the microspheres were sintered together.
These property enhancements have made MMCs attractive for structural applications at room temperature and at elevated temperatures. The magnitude of the enhancements in properties depend on many factors, such as, the:
In addition, all other factors which generally affect properties of metallic materials, such as the heat-treatment, fabrication, porosity level, etc., also affect the properties of MMCs.
Aluminium, magnesium, titanium, copper and their alloys can be used as matrix materials. For reinforcing purposes, a variety of compounds such as carbides, oxides, nitrides, borides, etc. and element such as carbon and boron, in various forms such as fibres, whiskers, platelets and particulates, have proven to be effective. High performance fibres and whiskers are manufactured by costly, energy-intensive processes which make them expensive, resulting in high cost to the finished MMC product. There are certain lower performance fibres and particulates, however, which produce moderate property improvements at low cost. The present invention relates to a reinforcement for low-cost MMCs incorporating an in inexpensive naturally occurring raw material.
Although, some naturally occuring materials such as natural graphite, mica and zircon sand have been examined for reinforcing application in MMSs, they are not recommended because the properties of the resulting MMCs are generally poorer than those of the matrix. An important aspect of this invention is the use of a naturally occuring mineral as the starting material for reinforcement of MMCs and other composites leading to enhancement in properties.
The geometries of reinforcing materials employed by MMC producers are typically fibre, platelet and particulate. The fibre geometry has a long dimension in one direction which is the preferred direction of reinforcement. Little or no reinforcing effect, sometimes even deleterious effects, are observed in the directions perpendicular to the longitudinal direction. The platelet geometry, in turn, imparts reinforcing effects only in the plane of the platelet which, again, is the preferred plane for reinforcement. Consequently, MMCs reinforced by fibres and platelets exhibit anisotropic properties. Randomly oriented fibres and platelets can restore isotropy but this is difficult to achieve in practice and unlikely to be maintained during forming operations.
Particle reinforced composites do not exhibit the anisotropic characteristics of fibre or platelet composites. Hence they are the most efficient for the production of isotropic MMCs, in addition to being low in cost. The use of the ceramic microspheres defined above as a reinforcing material offers, by virtue of its geometry, a high degree of isotropy to the MMCs.
The particulate materials presently used for reinforcing purposes have angular or irregular shapes. When reinforcements of such shapes are incorporated in a metallic matrix, the particles can act as stress raisers under the action of an applied stress. This can lead to premature nucleation of cracks with associated reduction in plasticity of the MMC. Reduced plasticity is reflected in lower elongation, toughness, formability and perhaps strength. The geometry and fine microstructure of the particulate material embodied in this invention is free of any stress raising features such that MMCs reinforced with the material have improved ability for plastic deformation.
Microspheres of the above type have now been determined to be suited for use as a reinforcement material in metals to form metal matrix composites as well as in other materials such as plastics, to provide improved strength and wearing properties. In making microspheres for this purpose it is important to ensure that the microspheres have properties which promote "wetting" which determines the extent to which the interface of the microsphere transforms the strengthening effect of the ceramic.
Good wettability between the matrix alloy and the reinforcing material is critical for the production of good quality MMCs. Many studies have shown that commercial reinforcing materials are not easily wetted by molten metals and alloys, and similar comments apply with equal validity to offer materials such as plastics. A good reinforcing material, therefore, should exhibit good wettability while being relatively inert to the matrix alloy. Such a reinforcing material is provided in this invention.